Conventionally, there is an electric toothbrush for generating vibration by rotating an eccentric shaft with a motor (refer to Patent Document 1). In such an electric toothbrush, a gravity center position of the eccentric shaft (a distance from a gravity center to a shaft center and a position in an axial direction of the eccentric shaft) is not changed. Therefore, neither resonant frequency nor vibration amplitude is changed.
However, there are cases where the resonant frequency and the vibration amplitude of the eccentric shaft are appropriately changeable for various reasons of obtaining feel of brushing at the time of brushing teeth, a brushing effect in accordance with a part of the teeth to be brushed, and the like.
Patent Document 1: Japanese Unexamined Patent Publication No. H 10-192054